So the difference is on the back, the FS model is a bit longer, the nesting with the silencer is slightly different and the last curvature is a bit different

I can understand that the curvature is different because the exhaust of the SF is not fixed in the same way as on the FE; On the fs there are spacers in order to prevent the wider tire from coming into contact with the muffler and so the central part had to be slightly different for the exhaust-silencer connection to be properly aligned

But I do not understand the difference in length ....

Finally I believe it is possible to mount a central part of FS on an FE or vice versa